The NFL is investigating claims the New England Patriots intentionally deflated balls to gain an advantage in Sunday's victory over the Indianapolis Colts that sent them to the Super Bowl.
The Patriots beat Indianapolis 45-7 at Gillette Stadium in driving rain.
Flat balls would, in theory, be easier to catch in wet conditions.
NFL spokesman Michael Signora confirmed the investigation on Monday and Patriots head coach Bill Belichick said they would "co-operate fully".
But Patriots quarter-back Tom Brady, who threw three touchdowns in the win, called the charge "ridiculous".
The home team have 36 balls available for testing before the match but at one point an official took one out of play during the game and weighed it.
If the Patriots were found to have deflated balls intentionally they could face a number of punishments - including losing a draft pick.
The investigation has been referred to as 'deflate-gate' in US media and on social media.
In the 2007 'spy-gate' affair the team were fined $250,000 (£165,000) and lost a first-round draft pick while coach Belichick was fined $500,000 (£331,000) after being found guilty of illegally filming another team's hand signals during a game.
The NFL's 32 teams are split between the American Football Conference (AFC) and the National Football Conference (NFC).
Patriots beat the Colts to win the AFC championship game and will face the Seattle Seahawks in Super Bowl XLIX in Phoenix, Arizona, on 1 February.
The Seahawks beat the Green Bay Packers on Sunday to win the NFC title game.